Sytest was using a wrong `history_visibility` for `invited`
(https://github.com/matrix-org/sytest/pull/1303), so `invited` was
passing for the wrong reason (-> defaulted to `shared`, as `invite`
wasn't understood).
This change now handles missing events like Synapse, if a server isn't
allowed to see the event, it gets a redacted version of it, making the
`get_missing_events` tests pass.
Really we should never have put the AS API bits under CS API to
begin with, as it is a completely different versioned spec: split
it out (which bumps the CS API % slightly).
Also, split out Device Keys into 2 extra sections: Device Key Backup
and Cross-signing Keys. The point of AWSY is for it to be easy for
anyone to see which features are implemented in Dendrite, and bundling
three distinct features under the umbrella of Device Keys was just
unclear.
